The present invention relates to communication apparatuses in radio networks, transmission control methods, threshold value control methods, programs, and radio network systems. Particularly, the present invention relates to communication apparatuses in radio networks, which defers transmission of transmission signals when signals, each having a received power exceeding a threshold value, are received.
The CSMA (Carrier Sense Multiple Access) scheme is applied in the access control scheme for communication apparatuses in the radio LAN (Local Area Network), typified by the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard.
Unlike the FDD scheme (Frequency Division Duplex scheme), the CSMA scheme utilizes the upstream line and the downstream line, with the same channel segregated. The significant concept of the CSMA scheme is that each communication apparatus performs carrier sensing (carrier detection). Transmission is suppressed when the signal having a received power exceeding a specified value (hereinafter referred to as a carrier detection threshold value) is being received. The intention is that communication apparatuses make mutual concessions of transmission to reduce interference between them. The conventional radio communication apparatus, which realizes the CSMA scheme, will be explained below.
FIG. 7 is a diagram illustrating the configuration of a conventional communication apparatus employing the IEEE 802.11 standard. Referring to FIG. 7, the communication apparatus consists of an antenna 110, a switch 120, a demodulator 130, a modulator 140, and a carrier detector 150. The switch 120, which is controlled by the carrier detector 150, selects the demodulator 130 or the modulator 140 according to the switching signal from the carrier detector 150.
The carrier detector 150 receives the signal received with the antenna 110 and then measures the received power thereof. The carrier detector 150 compares a predetermined carrier detection threshold value Rth with the measured received power. When the measured received power is larger than the threshold value Rth or a modulation intended signal (a transmission signal to be transmitted) in a transmission waiting state does not exist, the carrier detector 150 controls the switch 120 to connect the demodulator 130 to the antenna 110. If not so, the carrier detector 150 controls the switch 120 to connect the modulator 140 to the antenna 110 after the waiting for a random time. Moreover, using the transmission indication signal, the carrier detector 150 commands the modulator 140 to transmit the modulation intended signal. The modulator 140 notifies the carrier detector 150 of the presence or absence of the modulation intended signal, using a modulation intended signal presence/absence signal.
The reason for random time waiting is as follows. That is, when the communication apparatus changes from a carrier detection state (or a demodulation state) to a carrier non-detection state (or a transmittable state), other radio communication apparatuses distributed around the communication apparatus similarly change from the demodulation state to the transmittable state. Thus, if both the communication apparatuses simultaneously transmit their transmission signals, without the random time waiting, the two radio signals are superimposed, thus being received by other radio communication apparatuses.
The modulator 140 has a buffer (not shown). If the carrier detector 150 does not indicate transmission of a transmission signal, regardless of the transmission signal being received by the modulator 140, the modulator 140 stores the transmission signal into the buffer. After the transmission indication from the carrier detector 150, the modulator 140 outputs the transmission signal in the buffer to the antenna 110. Alternatively, transmission signals with low priority may be discarded without storing in the buffer.
As explained above, in the communication apparatus shown in FIG. 7, transmission stands by in the state where signals are being received from other communication apparatuses. This makes it possible to prevent interference to other communication apparatus, which is receiving the corresponding signals, before it happens. With all neighboring radio communication apparatuses that perform the waiting operation, alternate transmission can be performed so as to prevent interference between them.
FIG. 8 is a diagram illustrating the configuration of the conventional radio communication apparatus described in the patent document 1. Referring to FIG. 8, the communication apparatus consists of an antenna 210, a demodulator 220, a carrier detector 230, a transmission threshold value control/transmission controller 240, a transmission threshold value register 250, and a modulator 260.
The demodulator 220 and the carrier detector 230 accept the signal received by the antenna 210. The carrier detector 230 measures the received power of the received signal and outputs the measured power value to the transmission threshold value control/transmission controller 240. At the same time, the carrier detector 230 compares a predetermined carrier detection threshold value Rth with the measured received power and then outputs the carrier detection signal to the transmission threshold value control/transmission controller 240 when the measured received power is larger than the threshold value Rth.
The demodulator 220 detects the network ID included in the received signal and then outputs it to the transmission threshold value control/transmission controller 240. The transmission threshold value control/transmission controller 240 refers to the received power value and the carrier detection signal from the carrier detector 230 and the transmission threshold value held in the transmission threshold value register 250 and then outputs a transmission enable/disable signal to the modulator 260. The transmission threshold value control/transmission controller 240 varies the transmission threshold value held in the transmission threshold value register 250 based on the received power value from the carrier detector 230 and based on the network ID from the demodulator 220.
The transmission threshold value control/transmission controller 240 indicates transmission enabling or disabling to the modulator 260, as follows. That is, when the transmission threshold value control/transmission controller 240 receives a carrier detection signal, representing detection of a carrier, from the carrier detector 230, or when a measured received power exceeds a transmission threshold value, the transmission threshold value control/transmission controller 240 outputs a transmission enable/disable signal indicating suppression of transmission to the modulator 260. With the above-mentioned requirements not satisfied, the transmission threshold value control/transmission controller 240 outputs a transmission enable/disable signal indicating permission of transmission to permit the modulation and transmission of a modulation intended signal (a transmission signal to be transmitted) in the modulator 260 to the modulator 260 after waiting for a random time. When receiving transmission permission from the transmission threshold value control/transmission controller 240, the modulator 260 outputs the transmission signal to the antenna 210 after waiting for the random time.
The transmission threshold value control/transmission controller 240 controls the transmission threshold value variably, as follows. That is, when the network ID from the demodulator 220 matches with the network ID representing a network belonging to a local communication apparatus, the transmission threshold value control/transmission controller 240 varies its transmission threshold value. When the measured received power value is larger than the transmission threshold value held in the transmission threshold value register 250, the transmission threshold value is increased by a fixed value. When the measured received power value is smaller than the transmission threshold value, the transmission threshold value is set to be equal to the measured received power value.
As described above, the communication apparatus shown in FIG. 8 operates in such a way that a transmission threshold value is set to the minimum received power of a signal coming from the network or cell belonging to the concerned communication apparatus. Even if the signal is received from a radio communication apparatus belonging to a cell except cells belonging to the concerned radio communication apparatus, the transmission threshold value is not adjusted. For that reason, the received powers of received signals from a large number of radio communication apparatuses belonging to cells around the cell belonging to the local radio communication apparatus are less than the transmission threshold value. That is, radio communication apparatuses belonging to the same cell mediate to each other for transmission. However, such transmission regulation is not carried out between radio communication apparatuses belonging to different cells. Therefore, the phenomenon can be prevented that radio communication apparatuses belonging to different cells, between which mutual communications are riot required, may excessively suppress their transmission to each other.
The conventional threshold value control method described in the patent document 2 will be explained here. The threshold value control method, which is applied to communication apparatuses in radio LANs, controls variably the carrier detection threshold value to defer the transmission when the received signal has a received power higher than the carrier detection threshold value.
In the threshold value control method, the first radio communication apparatus first receives signals transmitted almost simultaneously from the second radio communication apparatus and the third radio communication apparatus. In this case, the distance between the first radio communication apparatus and the third radio communication apparatus is larger than the distance between the first radio communication apparatus and the second communication apparatus. If signals from the second communication apparatus can be decoded better, the first radio communication apparatus increases the carrier detection threshold value. If signals from the second communication apparatus cannot be decoded better, the first radio communication apparatus decreases the carrier detection threshold value.
Accordingly, when receiving the signal transmitted from a radio communication apparatus within the range where received signals can be better decoded, the first communication apparatus defers its transmission operation. It is prevented that the first communication apparatus defers its transmission because of reception of signals from a radio communication apparatus outside the decodable range.
[Patent Document 1]
JP-P1994-029981A (refer to pages 3 to 6 and FIGS. 3 to 14)
[Patent Document 2]
JP-P2001-217848A (refer to page 8 and FIGS. 12 and 13)
Wide area deployment in the so-called small zone scheme, where plural base stations or access points (APs) are deployed to provide a wide service area and this is the so-called small zone scheme, is performed. In this case, cells overlapped cause each radio communication apparatus to receive signals from cells belonging thereto as well as many signals from other cells. Hence, when the communication apparatus shown in FIG. 7 is used for each radio communication apparatus employing the small zone scheme, transmission suppression frequently occurs in each communication apparatus, so that a large decrease in throughput arises. On the other hand, the communication apparatus shown in FIG. 8 may be used to each radio communication machine employing the small zone scheme. In such a case, because each communication apparatus implements transmission suppression against interference only within the cells belonging to itself but does not sense interference signals from other cells, it can be avoided to suppress transmission excessively as degree as that of the communication apparatus shown in FIG. 7.
In the use of the communication apparatus shown in FIG. 8, communication apparatuses belonging to different cells do not make concession to each other in transmission, so that the frequency of generation of transmission suppression in each communication apparatus can be decreased. However, even in that case, there is the problem that transmission is excessively suppressed in a radio link within a cell. The problem will be explained below.
The radio link between the first mobile terminal being a communication apparatus located inside a cell and an access point (AP) being a communication apparatus configuring the cell has a propagation loss lower than the radio link between the second mobile terminal being a communication apparatus located outside the cell and the AP. Therefore, the radio link between the first mobile terminal and the AP indicates a higher interference immunity than the radio link between the second mobile terminal and the AP. As a result, the carrier detection threshold value of the first mobile terminal can be set to a higher value than the carrier detection threshold value of the second mobile terminal.
However, in the communication apparatus shown in FIG. 8, a carrier detection threshold value is a fixed value. That is, the carrier detection threshold values of the first and the second mobile terminals are set to the same value. Hence, the problem is that transmission is excessively suppressed even in the radio link having a high interference resistance within cells. Similarly, the communication apparatus shown in FIG. 7 has the above-mentioned problem.
As described above, the threshold value control method described in the patent document 2 can avoid the problem that transmission is deferred because of reception of signals from a communication apparatus outside the range where the communication apparatus can better duplex received signals. Therefore, the frequency of generation of transmission suppression can be reduced. However, the threshold value control method has the above-mentioned problem. That is, even when a communication apparatus transmits signals via a radio link having a higher interference resistance, transmission is delayed if signals from communication apparatuses within the range are received.